The present invention relates to the identification of the mold in which a vessel made from an optically transparent material has been molded. The mold in which the vessels are molded has an individual marking suitable for imprinting onto the vessel comprising relief-like elements disposed on a circular path.
In the manufacture of an article in a press mold, casting mold or blow mold, the deficiencies and errors of the mold are transferred onto the article. The result is, especially in manufacturing installations equipped with a number of identical molds and possessing a high production rate, that two important problems arise: the identification of that mold in which a specific article suffering from a defect was molded and the sorting out of all the articles which have been made in this mold, before the defective article was recognized or in the period between recognition of the defective article and replacement of the corresponding mold. The solution of these problems is especially important in modern installations for the production of glass vessels. The molds utilized in such installations are subjected to exacting thermal conditions and high mechanical loads and exhibit a correspondingly high rate of wear. Furthermore, such installations are equipped with a number of identical molds and are operated at a high production rate. Since it is only possible for the vessels to be examined for possible errors after they have passed through a tempering furnace, a large number of vessels have usually been molded in a defective mold, before the defect is discovered.
In order to solve both problems, the molds are furnished with a marking, which is transferred onto every article molded in them. This marking may simply be an ordinal number. A known device for reading the ordinal number designating the mold on a glass vessel is described in French patent application No. 74 21259 (AB Platmanufaktur). This device is adapted for reading ordinal numbers which are represented in a digital code. The individual code elements are constituted as dash-shaped, relief-like projections, which are disposed along a peripheral circle and preferably on the bottom surface of the vessel. To read them, at least one part of the peripheral circle is illuminated. The relief-like projections of the code elements then cause both the light passing through the vessel wall and also the light reflected from the vessel wall to be diffracted more intensely than that from a plane vessel wall. To read the code, the illuminated vessel is rotated about the center of the circle in front of a stationary reading device, and the changes in illumination of the reading device caused by the code elements are converted into electrical signals. In order to distinguish the illumination changes produced by the code elements from those which, for example, are caused by other markings formed on the vessel wall by mold joints or irregularities in the vessel wall, the evaluating device connected to the reading device is controlled by a synchronizing generator. For the synchronizing generator, a signal generator may be used, which generates synchronizing impulses which are proportional to the rotational speed of the drive apparatus for the vessel.
A preferred use of this device in the sorting line of an installation for manufacturing glass vessels is described in U.S. Pat. No. 3,923,158 (AB Platmanufaktur). In this use, the output from the device and the output from at least one test device of the sorting line are connected to associated inputs of a recording device. As soon as one of the test devices detects an unacceptable fault or an inadmissable number of faults, the ordinal number molded on the defective vessel and simultaneously or previously read by the reading device is indicated by the recording device and is stored or printed out optionally as desired.
The above-described device possesses a number of disadvantages, which severly restrict its practical use.
The evaluating circuit can only evaluate the read signals correctly when the code elements to be read pass before the reading device synchronously with the synchronizing impulses. For this to occur, two conditions must be satisfied. Firstly, the rotational speed of the vessels must coincide exactly with the rotational speed of the drive mechanism which triggers the signal generator, that is to say the vessel must not be subject to any slip relative to the drive device. This requirement cannot in practice be fulfilled with the high throughput speeds and rotational speeds encountered in the measuring and testing stations of a modern sorting line. Secondly, the angular interval between the code elements on the peripheral circle of the vessel must stand in an exactly defined relationship to the angular intervals between the markings utilized for triggering the signal generator and situated on the drive device. This requirement can only be satisfied in new molds. On account of the exacting thermal conditions and mechanical loading of the molds already mentioned, these molds are subject to unavoidable wear and small changes in shape, which adversely affect the angular interval between the code elements. Independently of the quality of the mold, changes in angle between the code elements can also be caused by uneven expansion and contraction of the vessel during tempering.
In the device described, the relief-like molded code elements are utilized as optical lenses, which image the light source upon the photoelement of the reading device. This necessitates a relatively high degree of accuracy in the shape of each code element, which cannot however be guaranteed on account of the wear of the molds already referred to.
Furthermore, in this device, the reading device is illuminated with practically parallel light, so long as there is no code element in the light path between the source and the reading device. This basic illumination is dependent upon the color of the glass of the vessel and the thickness of its wall, and the reading device accordingly produces a variable base signal, upon which the signal generated by a code element is superimposed. The absolute value of the two signals is therefore dependent not only upon the illumination of the reading device by a code element, but also upon the fluctuations in the base signal. The absence of a constant reference signal, against which the read signals can be set in a comparative relationship, renders the evaluation of the signals difficult and can indeed make this evaluation impossible.